Extrusion dies have been found to be useful in forming cellular or honeycomb ceramic substrates for use in catalytic converters utilized in the exhaust system of internal combustion engines. In order for such converters to function efficiently, it is necessary that the cells provide a substantially large surface area for catalytic material to react with the exhaust gases, and that the cell walls have a substantially thin cross-sectional dimension so as to provide a substantially large open frontal area and thereby reduce back pressure within the exhaust system. However, the thin walled structure must have sufficient mechanical and thermal integrity so as to withstand normal automotive impact and thermal requirements.
The extrusion of such honeycomb ceramic substrates may be accomplished through the utilization of monolithic billet dies such as shown in FIGS. 1-6 of U.S. Pat. No. 3,905,743, wherein the die is formed in a unitary die block by utilizing conventional machining and cutting techniques, electric discharge machining, or chemical machining. Generally, unitary die blocks are formed of a malleable material which not only facilitates ease of machining the same, but also provides a degree of elasticity to accommodate stresses and bending moments generated centrally of the discharge face during the application of high extrusion pressures. However, a major disadvantage encountered with such dies resides in their limited useful lives due to their poor wear characteristics. That is, the abrasive nature of the ceramic batch material, which is extruded through such dies, has a tendency to materially abrade and wear away wall portions of the discharge slots to such an extent that the resulting substrate becomes out of tolerance and the die must be discarded.
As pointed out in FIGS. 7, 8 and 9 of U.S. Pat. No. 3,905,743, and as set forth in U.S. Pat. No. 3,923,444, extrusion dies for honeycomb ceramic substrates may be formed from a plurality of elongated extrusion plates which are clamped together in a stacked condition to form a laminated extrusion die. A major advantage which can be achieved in utilizing such laminated extrusion dies resides in the fact that each plate may be formed of an extremely hard wear-resistant material, and yet be relatively easily machined due to the thinness of each plate as compared to the vast expanse of a billet die. Even though excellent wear characteristics are obtained with the use of laminated extrusion dies having die blades formed from such wear-resistant refractory hard metals as tungsten carbide, such dies, being of extremely brittle material, have a tendency to break under the tensile stresses created in the outlet face when under extrusion pressures.
The present invention overcomes the breakage problems encountered with laminated dies formed of hard, brittle, wear-resistant refractory materials, such as tungsten carbide, by applying a compressive force to the ends of the refractory hard metal blades, which force is directed parallel to the neutral axis thereof so as to virtually eliminate all tensile stress in the blades. Although the use of compression for supporting various objects has been known, as shown in U.S. Pat. No. 3,591,411, a particular mode of application is set forth herein which provides improved structural die performance.